A wide variety of fuel supply system are well known and widely used in modern internal combustion engines. In some instances, fuel is pressurized for injection by way of a so-called unit pump that can be mechanically actuated by way of an engine cam. The unit pumps are typically coupled with or part of individual fuel injectors, although designs are known where a single unit pump provides fuel pressurization for multiple injector units. Other fuel supply systems employ a common rail that stores a reservoir of pressurized fuel to be delivered to individual fuel injectors. Both general types of systems have certain advantages and disadvantages.
Mechanically actuated unit pumps or “unit injectors” commonly include a spill valve that can be opened and closed to vary the pressurization profile of fuel within the associated fuel injector. For example, a cam-actuated plunger in a unit injector can reciprocate to draw fuel into a plunger cavity when the spill valve is open and pressurize the fuel in the plunger cavity when the spill valve is closed. By varying the state of the spill valve the relative extent of pressurization and timing of pressurization of the fuel can be varied. An outlet check within the fuel injector which can be directly hydraulically controlled, or controlled based upon a pressure of fuel within the fuel injector, lifts to open spray orifices and enable injection of fuel. The rate of fuel injection can be varied in a relatively straightforward manner by selectively opening, closing, reopening, et cetera, the spill valve. Engineers have experimented with so-called injection rate shaping in unit injectors for many years.
Common rail fuel systems can present greater challenges to varying injection rate. Although timing and delivery can be changed from cycle to cycle, injection pressure generally cannot respond rapidly enough within an engine cycle for much rate shaping because injection pressure is tied to system volume and responsiveness of a high pressure common rail pump. In general, common rail injectors are limited to a single rate shape, which is typically a square injection profile at all delivery conditions.
It has been observed that varying rate shape can be desirable regardless of fuel system type in that combustion properties such as combustion efficiency and emissions profile can be advantageously manipulated if rate shape can be controlled. While some degree of rate shaping can be achieved with precise control of the outlet check in a common rail system, such an approach can undesirably affect spray characteristics of the exiting fuel. U.S. Pat. No. 7,111,614 is directed to a single fluid injector with rate shaping capability. In the '614 patent, rate shaping is accomplished by way of a valve operably coupled to an electrical actuator and movable between a high pressure seat and a low pressure seat. Movement of a needle valve for injection and movement of an admission valve for varying injection rate are both apparently accomplished by way of movement of a control valve member with an electrical actuator.